Sometimes simple imitation is the best way to slow your student down. If she’s having trouble slowing down because she doesn’t feel the music when it’s played slowly, this may be the perfect solution for her.
Demonstrate a short section at a slow, practice-appropriate tempo.
Play the same section again, this time asking her to play along with you. (Play on a second piano or in a different octave.)
Repeat this until she is playing with you successfully, then move to the next section and start the exercise again.
You can record each section at the slow tempo for her to listen to and play along with at home if you feel she will need that extra helping hand.